A mask assembly and a thin film deposition method using the same are provided.The mask assembly includes a mask frame including first to fourth sides. The first to fourth sides form a rectangle. Inner sides of the rectangle define a window. The mask frame has a plurality of substrate seating portions provided to project toward the window from at least two corners. The two corners face each other in a diagonal direction. The mask assembly includes four corners positioned where the first to fourth sides of the mask assembly meet each other. A mask includes a plurality of openings for deposition. The plurality of openings are arranged to correspond to the window.